Demos: 2B-10 Archimedes II

This demonstration illustrates the concept that the buoyant force is equal to the weight of the fluid displaced. A block is completely submerged in water whose level is just at the overflow point. The displaced water is caught in a small beaker. After noting the scale reading when the block is submerged, the caught water is poured into the open-top can and the scale reading returns to its initial value.

Directions: Preparation: Fill the overflow beaker until water begins to come out the spout. (Catch it in the small beaker.) After the water has stopped dripping, discard the water from the small beaker and place the beaker back under the spout. After recording the scale reading, VERY SLOWLY lower the block into the water until it is just submerged. Wait a bit until the water has finished dripping from the spout. Record the new scale reading. Then take the beaker and pour the caught water into the open-top can. The scale reading will return to its initial value.

Suggestions for Presentation: Presumably the students have seen a demonstration showing that when an on object is submerged in a fluid, its apparent weight diminishes. Ask the students what determines how much the weight is reduced? Does it depend on the material of the object being submerged? After establishing that the buoyant force is equal to the weight of the displaced fluid, argue that if the displaced fluid is separated out and poured in the cup, the weight that was lost will be added back and the system should return to its initial state.